The present invention relates to an apparatus for selectively electroplating apertured metal or metallized products obtained by stamping or etching. In the invention, a fully closed photoresist layer is electrophoretically applied to the apertured products, and after drying of the photoresist layer, the apertured products are covered with at least one photomask at desired locations, followed by exposure of the products covered with (a) photomask(s), after which the photoresist layer is removed, by means of a developing process, from the parts of the apertured products to be electroplated after removal of the photomask(s), while the remaining part of the photoresist layer on the apertured products serves as a mask for the metal products upon electroplating those parts of the apertured products from which the photoresist layer has been removed.
In the industry use is frequently made of metal or metallized products, which for technical and/or economic reasons must be selectively electroplated, mostly with a precious metal. Generally it is required that the electroplating must take place as accurately as possible on the intended places, since plating of non-intended parts may lead to malfunctions when the product is being used and/or to undesirable consumption of the costly plating material.
FIG. 1 shows a typical example of such a product in the shape of a metal frame which is known per se, a so-called leadframe 1, with parts of leadframes contiguous thereto. Such leadframes are usually stamped or photochemically etched from a metal band or strip or obtained by photochemically etching of a metal foil applied to a film of plastic material.
FIGS. 2 and 3 likewise show products known per se, which are interconnected in endless product bands or strips, FIG. 2 showing an example of products 2 used for transistors and FIG. 3 showing an example of products 3 used for contact systems.
These products have parts, which are indicated by hatched portions in FIGS. 1-3, which are to be plated with a metal. It is also possible that only a small part of a surface must be plated, for example the part indicated by black planes in FIG. 3. Furthermore these products are provided with a larger or smaller number of apertures 4, which at least partially surround the areas to be plated with a metal.
Although it will also be possible to use the apparatus according to the invention to be described hereafter for products such as those shown in FIGS. 2 and 3, the invention will be explained in particular with reference to its use with the so-called leadframes, since these products are of a generally very vulnerable, complicated structure, while in particular with these products very strict requirements are made with regard to the plating of a metal solely on the intended places, while using a minimum amount of plating material.
As already noted such leadframes, which are initially interconnected in elongated bands or strips, are usually made by etching or stamping. In order to be able to carry out such plating of metal products as accurately as possible, there have been quite a few developments over the years, as for example described in U.S. Pat. Nos. 3,746,730, 3,819,502, 3,855,108, 4,376,017 and 4,493,757, and also in European Patent Applications Nos. 0055130 and 0382283. As is apparent from these publications the bands or strips may thereby be continuously or intermittently transported through the processing apparatus, while during the intended electroplating mechanical masks, such as belts of a flexible material moving along with the products or pads of a flexible material, which are intermittently pressed against the products, cover those parts of the products which must not be plated.
When using a leadframe 1 as shown in FIG. 1, a chip is usually placed on a platform 5 and connected to the ends of the leads 6 located nearby the platform by means of gold or aluminum wires. Then the platform and the chip and the ends of the leads 6 connected to the chip are encapsulated in plastic material and the surplus parts outside the package of plastic material are cut from the leadframe shown, while the parts of the leads 6 projecting from the package of plastic material will be used for connection into an electronic circuit, such as for example, a printed circuit or the like.
The embodiment of the leadframe diagrammatically illustrated in FIG. 1 is of very simple construction. In practice leadframes having more than 200 leads are sometimes used. An example of a leadframe 1a having 208 leads is shown in FIG. 4. Usually such products having a very complicated structure and comprising a large number of leads are not manufactured in the form of elongated bands to be wound on coils, but in the shape of strips having a length of 150-240 mm.
In addition to the increasing integration of functions at chip level, with the increase in the number of leads per chip resulting therefrom, there is a tendency to reduce the thickness of both the metal used for the leadframe and the above-mentioned package of plastic material, in which the chip and the leads on the chip are encapsulated. It is already being attempted to reduce the thickness of the leadframe material from the usual 0.25-0.35 mm to 0.08-0.15 or less, while an aim is to achieve a package thickness in the order of .+-.1.2 mm instead of the 3.0 mm of the packages used so far.
This miniaturization trend may be expected to continue.
Increasingly strict requirements are made of the accuracy of position and form of the metal selectively plated onto the products, also with a view to the reliability of the end products in which such leadframes provided with chips are used.
It is no longer possible to realize these strict requirements with the mechanical masks that have been used so far, wherein use is made of upper and/or lower masks, of which at least those parts that come into contact with the products are usually made of resilient material, and wherein at least one of the masks is furthermore provided with passages in accordance with the selectively plating to be carried out on the products.
Thus FIG. 5 schematically shows a sectional view of a number of leads 6, which are clamped between an upper mask 7 and a lower mask 8, as is usual when the above-described apparatuses are used for electroplating.
As is apparent from FIG. 5, channels 10 are thereby present between the cut edges 9 of the leads, in which an electrolyte may flow during the intended electroplating, so that not only an intended part 11 of a lead 6 (FIG. 6) will be plated with a metal, but also, at 12, the cut edge 9, as indicated by the hatched portion in FIG. 6.
FIG. 6 also indicates, by means of lines A--A and B--B, the boundary of the package of plastic material referred to above. In practice it often occurs that the undesirable metal coating, usually consisting of silver, which is formed on the cut edge 9, extends beyond such a package of plastic material, as indicated by a cross-hatched portion 13. The fact that the plating material, silver, projects from the package of plastic material involves the risk of a short-circuit among the leads as a result of the migration of silver over the surface of the plastic material.
Furthermore the material of the aforesaid package bonds much better to the base material than to the plated material, so that also in view of the bonding the presence of plating material on non-functional places is disadvantageous.
Also the platform 5 (FIG. 1) must be provided with an intended metal coating only at the side on which the chip is provided. When using the techniques comprising mechanical masking systems that have been used so far, it becomes apparent, however, that in particular with the, complicated leadframes made of a very thin material, using the masking techniques that have been usual so far, electrolyte will seep through and along the cut edges of the platform to that side of the platform which faces away from the side on which the chip is to be provided, so that in the end product the side of the platform 5 facing away from the chip is also at least partially plated with a metal, as indicated by a hatched portion in FIG. 7. As already stated, this has an adverse effect on the bonding of the package of plastic material encapsulating the central platform with the chip, so that it is desirable to avoid such plating of the rear side.
Besides the problems referred to above there is also a great risk of mechanical damage, such as bending of the platform and/or of the leads and the indexing holes when using the mechanical masking systems that have been usual so far with the above-described, particularly vulnerable leadframes. This may already be caused by pressing the mechanical masking systems against the very thin products with the required force. Furthermore the vulnerable parts of the leadframes tend to stick to the masking systems when the mechanical masking systems move in a direction away from the leadframes, as a result of which the further transport of the leadframes is interfered with and successive products will come into contact with each other.
In order to be able to meet the increasingly strict requirements with regard to the accuracy with which metal products are to be plated, the applicant has contemplated covering the metal products with dry films of a photosensitive material and locally exposing the films in accordance with the intended plating, in order to be able to subsequently uncover the metal product again at desired locations, by means of a developing process, with a view to carrying out the plating. When such films are applied to either side of apertured products it becomes apparent, however, that after exposure and developing the films present on the front and rear sides of the product do not properly abut each other between the apertures, resulting in channels being formed, while also the cut edges of the apertures provided in the product are not masked in the product surface uncovered by exposure and developing. Consequently, the cut edges of the apertures in the uncovered surface and at least part of the channels present between the upper and lower films are undesirably plated in the final plating process, in a similar manner as described above with reference to FIGS. 5 and 6.
Furthermore it has been contemplated to immerse the products in or spray them with liquid photosensitive materials and to expose desired places of the photosensitive materials in a similar manner as described above. It became apparent thereby, however, that after drying of the photosensitive layer the photosensitive layer had withdrawn to such an extent at sharp edges and burrs possibly present on the edges, that the photosensitive layer remaining behind at that location was porous and provided insufficient protection when plating was carried out following exposure and developing, so that plating also took place on undesirable places along the insufficiently protected edges and/or burrs, with all the disadvantages attached thereto.
The disadvantages resulting from the use of mechanical masking systems and of photosensitive foils or liquid photoresists can be avoided by using the method referred to in the introduction, wherein, as described in European Patent Application No. 0507043, a photoresist layer is electrophoretically applied on the apertured products. As described in EP-A-0507043, the apertured products are immersed in a photoresist bath for that purpose, whereby, dependent on the photoresist being used, the product is used as an anode or a cathode. The product is kept immersed in the bath thereby until the desired photoresist layer has grown on the product, which becomes visible in that the current applied has become zero or practically zero. In order to promote the formation of a satisfactory coating thereby, the photoresist bath is preferably vibrated. The electrophoretic resist layer applied, fully encloses all parts of the apertured products, sharp edges included.
Immersing the product, connecting the product to a power source, keeping the product immersed in the bath for some time and subsequently disconnecting the power source and removing it from the bath again all add up to the fact that this is a very time-consuming method.